A typical structure and manufacturing process of the alkaline-manganese cell which is most popular and mass produced presently among various types of the alkaline cell are first explained in the following. FIG. 1 shows a vertical cross-section of AA-size LR6 type cell.
A mixture comprised of manganese dioxide powder and graphite powder is molded into a cylindrical hollow pipe, and plural of such molds are stacked in an axially aligned form. After these molds acting as a cathode 1 are inserted into a cell case 4, the cathode is pressed against the inner wall of the cell case 4 which acts as a positive terminal.
Then, a cylinder-shaped separator with a bottom, 3, is inserted into a cylindrical space provided within the cathode 1, and alkaline electrolyte is supplied into the center space of the separator 3 in order to wet the separator 3.
A gel-anode 2 comprised of zinc powder dispersed into alkaline electrolyte of which viscosity is enhanced by means of a viscous agent such as carboxy methyl cellulose, is then supplied into the inner space within the separator 3. After a collector 6 integrated with a bottom plate 5 acting as a negative terminal, insulating washer 7, and a gasket 8, is inserted into the gel-anode 2, the cell case is sealed, and is packaged with a jacket 9.
However, within the alkaline-manganese cell manufactured by the process shown in above, growth of oxide film over the area contacted with the cathode may take place within the cell case, and by this, the electrical resistance between the cell case and the cathode is increased gradually.
Since this increased resistance and thus increased IR loss within the oxide layer invites cell voltage drop and deterioration of discharge characteristics, these should be processed particularly when the cell is operated a heavy load.
In order to solve this problem of increased ohmic resistances between the cathode 1 and the cell-case 4, a method providing plural axially protruded ridges on the inner wall of the cell case 4 at the area contacting with the cathode mold, or, a method such as disclosed by the Japanese Patent Publication Sho 42 (1967)-25145, providing a conductive film 10 on the inner wall had been developed and employed.
The most effective among these methods was found to be a method by which an electrically conductive paint made mainly of graphite and/or carbon black powder is applied and dried in order to form an electrically conductive film on the inner wall of the cell case 4.
FIG. 2 is a partially enlarged view of the cross-sectional area contiguous to the cell case 4 wherein the specified numbers are identical with the corresponding numbers shown in FIG. 1.
Although either one of the conventional methods shown below had been employed in order to dry the before-mentioned conductive paint, problems shown below had been found.
(1) Method to dry at room temperature
This is advantageous since no particular manufacturing equipments are required. However, since at least one day has to be allowed before complete drying of the paint, this process has to be conducted at a dedicated place other than the alkaline cell manufacturing area. Moreover, the wet paint coated on the inner wall of the cell case trailed down very often producing streaks of swelled conductive paint or a conductive film with uneven thickness.
Since these swelled paint streaks may easily be peeled off at the time when the cathode molds are inserted into the cell case, this had been considered as one of the major causes producing uneven load characteristics of the cells. Furthermore, since the thermosetting resin was found improper to be used as the paint binder resin, the types of binder resin had been limited.
(2) Method to dry by hot wind
This is a method by which the cell case painted with conductive paint is placed in a hot wind oven heated at about 100.degree. C. in order to purge the paint solvent or in order to cure the thermosetting resin such as epoxy resin used as a binder of the paint if such paint is used.
However, even with this heating method, it takes a period of one to ten minutes before the coating paint layer is completely dried or cured. Thus, a fairly large batch system or a fairly large manufacturing system formed around a conveyor line provided with a residence time has to be provided for this.
Moreover, in a case where a deep cell case, for example LR6 (AA-size) or LR03 (AAA-size), having a large aspect ratio between the inner diameter and the depth from the flange to the bottom, has to be painted, complete elimination of the paint trailing phenomenon is virtually impossible, so the complete elimination of variation in the load characteristics of the cells.
(3) Method to dry at decompressed atmosphere
This is a batch method using a decompressed chamber instead of hot wind oven. Since boiling of the paint solvent may take place in the decompressed chamber, this may result in poor adhesion of conductive layer onto the cell case. Moreover, the batch system is not advantageous in respect of the productivity.